The present invention relates to apparatus for temporarily rendering rigid a product of soft material of very elongated form before said product runs past and in front of a working station, of the kind in which said product is cooled down to the stiffening temperature in a liquid nitrogen refrigerating tunnel, with a refrigerating duct of said tunnel comprising an upstream part in the direction of traversal of said product which has an internal transverse outline having greater dimensions than those of the internal transverse outline of a downstream part of said duct. Hereinafter such a method will be referred to as "of the kind described."
It has been observed that the application of a liquid nitrogen tunnel lent itself more satisfactorily to this kind of operation by virtue of its very low temperature (-196.degree. C.) than the mechanical refrigerating appliances which do not ensure the absence of moisture at the location of the processing station and which, by virtue of this fact, commonly require auxiliary drying means.
The application of liquid nitrogen is of greater interest than that of carbon dioxide, whether in solid or liquid form, the temperature of which is inadequate to secure rapid cooling, and the handling of which in the case of solid carbon dioxide requires the use of labour for its immersion in a refrigerated bath commonly consisting of methanol. Although the conditions of application of liquid nitrogen lend themselves remarkably well to the technique for cold-stiffening a soft product of rubber or plastics material, a high cost frequently represents an obstacle, which derives from a comparatively high consumption combined with an extremely low thermal efficiency.
It is an object of the invention to increase the thermal efficiency of such rigidifying or stiffening operations utilising liquid nitrogen, in such manner as to render liquid nitrogen economically competitive with the other sources of cold commonly utilised. Another object of the invention is to provide a refrigerating or freezing tunnel of uncomplicated structure, in respect of the heat exchangers in particular, and furthermore in automating all the functions of such freezing tunnel.